In pressure units, the equilibrium constant has the same form as for gas-phase reactions, as stated in Equation 3–8 . The pressure equilibrium constant is directly related to the net Gibb’s free energy of the reaction, as determined by the thermodynamic properties of the species involved in the reaction. In deriving the equilibrium constant in concentration units, however, the surface state as well as the gas state must be taken into account. For surface reactions, then, takes the form:

(4–6)

where denotes a pressure of 1 atm, and is the standard-state surface site density of site type . The sum in the first exponent runs only over the gas-phase species, and the sum in the second exponent runs only over surface species in surface phase .